Obolellida is a small, extinct order of
inarticulate brachiopods that existed from the early to middle
Cambrian period. The relationship of the Obolellida with other inarticulates is unclear, and were previously grouped together with the
Siphonotretacea, before being given their own order. One representative,
Mummpikia, has been linked to the origin of calcitic shelled brachiopods more generally, hinting that obolellids may be paraphyletic. Trematobolidae includes taxa such as
Alisina, whose soft-part anatomy is partly known.
Anatomy The shell is typically
impunctate,
biconvex, and oval or subcircular in shape. They are like other inarticulates in that, as with the
lingulids, the shell has no hinge, at least in the earliest examples. It is thought they may have used a hydraulic mechanism. However they later seem to develop a primitive articulation, in which some used a levator mechanism for opening the shell. At the posterior is a ventral valve with a defined flat-shelf. The
pedicle opening can be positioned at various positions. == Naukatida==